The return on damage for Hecarim was never really worth it to build tons of MS anyway.
His passive is a nice bonus, but it shouldn't be built around.
I honestly don't think this will be that noticeable when all's said and done.
at level 18, hecarim converts 25% bonus movement speed into attack damage. once again, with runes(+4.5%), masteries(+2%), trinity force(+8%), reverie active(+40%), force of nature(+8%), tier 2 boots(+70), ghost(+35%) his e(+75%), and maybe, MAYBE ghostblade (+20%) do the math and tell me that isnt noticable when he hits the enemy carry with his e and all those bonus movement speed items + effects, summoner spells.
not to mention, if a support also has SR, or even janna who gives an extra 3%, or maybe a lulu+galio+orianna+kayle etc who gives him a movement speed boost... it really does add up